Getting to Know Magnet Materials – Your Questions Answered
Magnet materials are key to ensuring the application they’re used for is working to its optimum capability, and with so much information out there, it can prove to be quite the task finding the right one for you and inevitably there are many questions to be asked.
At Eclipse Magnetics, we have a team of experts on hand to help you find the right magnetic solution for your business, but if you’re in the early days of finding the right material for your application, then we’ve compiled a useful list of commonly asked questions to help you understand the basics of magnetic materials and their functions.
Types of Magnets
Three types of magnets exist, permanent magnets that produce a magnetic field without the need for any electrical force, temporary magnets that only act as magnets when attached to a second magnetic source which enables them to lose magnetism when the source is removed, and electromagnets that require an electrical current to act as a magnet.
There are many types of permanent magnet materials that are available, including Ferrite , Rare Earth Magnet Materials (Neodymium Magnets and Samarium Cobalt ) and AlniCo, each with their unique characteristics. The different materials each have a family of grades that have different properties to each other, though based on the same compositions.
What is the Difference Between Ferrite Magnets and Rare Earth Magnets?
Rare earth magnets are magnets that are composed of the scientific group of rare earth elements. The most common of these are Neodymium Magnets (neodymium iron boron, NdFeB) and Samarium Cobalt (SmCo). They are regarded as, size for size, the most powerful magnets available.
The neodymium magnets are more powerful than the samarium magnets at ambient temperature but the difference in output between each type reduces and at around 150 to 180 degrees C (Celsius) the SmCo magnets start to be the better performers and, when the highest temperature rated NdFeB grades are limited to 220 to 250 degrees C the SmCo grades are fine up to 300 or 350 degrees C with a recent samarium magnet addition capable of up to 550 degrees C.
Ferrite magnets are also known as ceramic magnets and are a type of permanent magnet made up of the chemical compound ferrite, consisting of ceramic materials and iron oxide. With a low production cost and a high heat resistance (up to 250 degrees Celsius), they are among the world’s most popular magnets for a wide range of applications, especially motors and generators. Ferrite magnets also offer excellent corrosion resistance (sometimes being referred to as ‘magnetic rust’ due to the iron already being in an oxidised state).
Rare earth magnets offer some of the strongest magnets in the world (great for power dense applications, such as high performance drive systems), but are more expensive, require coating (NdFeB magnets need a corrosion protecting layer such as NiCuNi or Zn) and are less resistant to heat (standard NdFeB grades).
What are Rare Earth free Magnets?
Rare Earth free magnets (RE free magnets) are basically magnets that do not contain Rare Earth elements. The term came about due to price fluctuations of the rare earth elements causing an interest in alternative topologies in electric drive systems (permanent magnet motors, PM drives, EV drives) and PM generators (PM = Permanent Magnet).
This usually meant the rare earth magnets used in motors and drives and generators being replaced usually by ferrite magnets. However, it is very difficult to replace the small volume of the motors that can be achieved due to being able to design in high power density into the machines. RE free motors tend to be a bit larger and have a higher cost from being larger, so there are trade-offs to consider, and the increase in demand for ferrite magnets that it caused had increased the price of ferrite magnets too.
A Rare Earth free motor (RE free motor) simply is a magnetic motor system that doesn’t contain NdFeB or SmCo magnetic materials.
How is an Electromagnet Different from a Permanent Magnet?
An electromagnet requires an electrical current to enable its magnetic field. Electricity and magnetism are very closely related and is demonstrated perfectly by the electromagnet. It has the same properties as an ordinary magnet, such as a north and south pole, and it attracts steel and iron. It differs from a permanent magnet simply because it can be switched on and off.
The electromagnet is created by a coil of wire wrapped around the core material. When electricity flows through the wound circle of wire, it creates a magnetic pull. One end of the wound wire coil acts as the North pole and the other end the South pole. The more loops of wire wound around the core, the stronger the magnet will pull.
What are Magnet Ratings?
Magnet rating, or magnet grade, is a measure of strength for a particular magnet. For example Neodymium magnets exist as grades such as N42, N52 or N42SH. For NdFeB and SmCo the higher numbers indicate a stronger magnet. The number actually comes from the Maximum Energy Product of the material (BHmax), stated in MGOe (Mega Gauss Oersteds). It represents the strongest point on the magnet’s demagnetization curve, or BH Curve. Taking NdFeB as the example, the pull force (or ability to provide magnetic output) from a magnet varies with the grade or N number, for example, as the N number increases, you will get an increase in the potential pull force that magnet can achieve. It should be noted that as the BHmax increases, so too does the Br of the magnet and this value is used to help calculate pull forces.
Other magnets such as ferrite and alnico also have BHmax values that can indicate the performance capability of the magnet (likewise the Br values for each grade) – unfortunately depending on the naming convention used with the grade of magnet, the name of the grade does not always correlate easily with the magnet’s ability to provide a high magnetic field output. So you should check the Br and BHmax values of grades if you want to get an indication of possible magnetic performance.
How do you Measure Magnet Strength?
This can be done by pull force or magnetic field strength measurements.
Pull force is the method of testing a material’s magnetic field when looking for the strength of a magnet. It is determined by the amount of force needed to pull the magnet away from either steel or another magnet. The pull force will depend on the size and shape of the magnet, its magnetic circuit and the environmental conditions. This is why pull forces in catalogues are often given as maximum possible values – the performance you will get is application specific – Eclipse Magnetics can help you with this.
Field strength values are measured in air gaps and have units of Gauss or Tesla – higher values indicate a more powerful magnetic field is present. Again, the magnitude of measured field is application specific in that it will depend on the size and shape of the magnet, the magnetic circuit and the environmental conditions. Eclipse Magnetics can help you get the most magnetic field from your application.
The maximum energy product (BHmax) of a magnet is measured in Mega Gauss Oersteds. If you can design a magnet to work at its BHmax value, you are using the smallest magnet volume possible to meet the application requirements. This is great is the environmental and magnetic circuit conditions allow as a smaller magnet is less weight, less space and less cost. But, for materials like alnico, alnico magnets working at the BHmax are easily demagnetised by external factors – so alnico magnets are deliberately made longer to force the working point higher up the BH curve to give safety from demagnetisation (because alnico has a lower Hc and Hci/Hcj than other magnetic materials). Eclipse Magnetics specialises in helping you design your magnets to operate better in your applications.
How Does Temperature Affect the Strength of a Magnet?
Magnetic materials go through a change in flux density as their temperature increases or decreases. Magnets will usually show an increase in magnetic strength output as ambient temperature drops. Likewise, magnetic output reduces as the magnets are heated up. The temperature range of the application will determine which grade(s) or magnet are better suited to your application (for example, very cold conditions do not always favour ferrite magnets).
All permanent magnets have averaged temperature coefficients for Br and Hci/Hcj that project their likely variation in magnetic output as the magnet temperature changes allowing you to start to predict how the magnet performance changes for every degree change in temperature. Whether the lost performance is regained upon cooling is dependent on the magnet materials used, the magnetic circuit and the environmental conditions (some changes are recoverable, but take the magnet too far and the losses become permanent/irrecoverable).
If a magnet grade were to be demagnetised by too much heat or cold, it may be possible to change the grade to reduce or eliminate that irrecoverable loss - Eclipse Magnetics can help you determine if a better magnetic solution exists and, if it does, Eclipse Magnetics can provide you with magnet samples to test the improvement we recommend.
So, is Designing with Magnets a Science?
Yes, designing with magnets does require an understanding of magnet theory but you have to take into account the application, the total magnetic circuit and also the environmental conditions. This is what Eclipse Magnetics does each day – we make magnets work to meet the intended purpose. Our combination of skilful Engineers, Technical experts and knowledgeable Sales Teams provides Eclipse Magnetics with a perfect ability to help guide you through all your magnetic requirements.
Get in Touch Today
If you would like to discuss your specific magnet needs with our team of experts, why not get in touch today? We offer free consultations to understand your requirements and will tailor a solution suited to your business. Click here to find out more.
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